Low density polyethylene (LDPE) is the least expensive polymer for the production of sheets and films and it is easy to produce films of from 30.mu. to about 500.mu. thickness and of a width of up to about 16 m by extrusion blowing. PE is quite transparent to visible light and to photosynthesis active radiation (PAR). Greenhouses covered with PE films lose much energy by radiation at night and frequently such greenhouses must be heated at nighttime.
The near infra-red (NIR) comprises 40% of the sun's radiation reaching the earth and it contributes to the heating up of the inside of greenhouses. In spring and winter crops in temperate climates, over-heating is a serious problem as it adversely affects certain plants. The energy in the visible range is adequate for crops in greenhouses.
The ratio of PAR/NIR in long day crops regulates the growth of plants such as tomatoes and cucumbers. PAR in the 0.4.mu. to 0.7.mu. range, and particularly the red 0.64 .mu. to 0.7.mu. range and the NIR range of 0.7.mu. to 1.5.mu. particularly at about 0.73 .mu. are mutually antagonistic. A high PAR/NIR ratio delays maturation and results in a high chlorophyl and protein content of crops, see Naturwiss 63, 530 (1976) and Am.Rev.Plant Physiol 23 293-334 (1972). Various polymeric films incorporating inorganic IR absorbing materials in fine particulate form are known: French patent No. 1574088 (silicasilicates), U.S. Pat. No. 4,179,547 (sodium metaphosphates) and U.S. Pat. No. 4,134,875 (alunite/aluminum hydroxide).
Films containing magnesium oxide are also known: they have an effect in the IR, but none in the NIR which is the main range dealt with by films of the invention.
Alunite/aluminum hydroxide containing sheets tend to deteriorate, the particles absorb IR radiation in the polymer matrix and their heating up over prolonged periods of time results in a deterioration of film quality. U.S. Pat. No. 3,857,807 discloses thermoplastic films for controlling plant growth. It contains certain additives which prevent a substantial part of PAR from passing through the film while transmitting in the NIR. Films for agricultural use which absorb in the IR have been reported. Such films transmit in the PAR and absorb in the 7 to 15.mu. range given off by soil and plants at nighttime in greenhouses thus decreasing heat losses by radiation. The IR and NIR ranges are quite different in their effects.